This invention relates generally to power generation and, more specifically, to programmable three-phase output voltage and frequency.
In most power generation applications, synchronous motors are driven to generate AC power. When so used, the frequency of AC power output is dependent upon the process used to apply torque to the synchronous motor. Constant torque at standardized values rarely exists in nature to precisely turn a synchronous machine. The rotational rate or angular velocity varies greatly. Because angular velocity is proportionate to the resulting frequency and voltage of AC power, where the angular velocity of the torque source is variable, the frequency and voltage of the resulting power is variable.
Randomly variable frequency AC power is not very useful. It is impossible to synchronize such power with a power supply network in a commercially practical manner. Such power cannot drive most applications designed for 60 Hz. line supply voltages. Few applications can tolerate such frequency variability.
To overcome the frequency variability in power generation, the solutions have been of two types, input and output solutions. Input solutions are mechanical and govern the power transfer to the synchronous machine. Output solutions condition electrical power garnered from the synchronous machine.
Traditionally, design constraints on power processing have required selection of components based upon the peak power to flow through those components rather than to design around the mean as in DC power systems. Periodically recurring peaks in the voltage and current waveforms for each phase develop recurrent power transfer well above the mean. Nonetheless, the power drawn from such a system as an aggregate is constant. For instance, the total power drawn from a balanced three-phase source by a balanced resistive load is constant. That is
Pt(t)=(V2/R)[sin2xcfx89t+sin2(xcfx89t+xcfx86)+sin2(xcfx89t+2xcfx86)]
=1.5(V2/R)xe2x80x83xe2x80x83(1)
where Pt=time value of power
V=peak line voltage
R=load resistance (per phase)
xcfx89=source frequency
xcfx86=27xcfx80/3
This fact is exploited in the cycloconverter at discrete frequencies. Small fluctuations in frequency are passed to the output. This need not be the case. The power transferred from source to load is not a function of time. The transfer of power can be accomplished without storing energy within the processor between input voltage cycles.
Because the power is independent of time and therefore phase, there exists a means, by aggregating the outputs from the phases to subject several of the components of the system to constant rather than varying power over time. Modulating the waveforms as the inventive apparatus does, xe2x80x9cknocks the corners offxe2x80x9d of the lower frequency waveforms as they come off the generator. In doing so, the inventive apparatus removes the need to accommodate the peaks of instantaneous power. The inventive system presents a system producing well-formed power output. Without converting and inverting power to and from DC power, the inventive system comprises smaller transformers and power storing filters than conventional means.
The traditional input solution to electrical power generation in applications such as aircraft has been through the Constant Speed Drive (CSD) coupled to a generator providing, for example, 115 VAC with three-phase power at a constant 400 Hz. In more recent times this arrangement has combined the CSD and generator into an Integrated Drive Unit, or IDU. With a constant frequency power output this has been a creditable solution, albeit expensive to buy and to maintain.
More recently, the output solution has been the Variable Frequency (VF) and cycloconverter systems. Cheaper of these two options, VF presents the load with power such as 115 VAC, three-phase power but only has a distribution capability at a frequency proportional to the engine speed. For a turbofan engine, for instance, this is usually 2:1. However, because of the wide range of frequency variation, power conditioning would be essential for almost all cases and, when added to the procurement, the additional cost of attendant motor controllers becomes prohibitively expensive.
In recent years, power has been generated with a Variable Speed Constant Frequency (VSCF) cycloconverter. A cycloconverter is a power electronics device designed to provide a variable voltage, constant frequency AC drive in a one stage operation, suitable for supply to an AC motor. These devices work by generating very high frequency three-phase power then selectively drawing voltages from the peaks of the three phases in a manner to construct rough approximations of lower frequency waveforms.
While a VSCF system unit (cycloconverter) does have the ability to produce AC and DC simultaneously, it does not produce clean waveforms. The voltage regulation is accomplished by a series of magnetic amplifiers, transformers, and bridge rectifiers. The VSCF drive uses a simple drive system and lets the alternator produce an electrical supply that is frequency wild, i.e. not well-controlled, which is then shaped by a solid-state electrical unit. Nonetheless, the resulting waveform includes several harmonics that impart an imaginary component to the power and may interfere with the function of the load.
Still another means of generating constant frequency power from a variable source of torque is based upon converting and rectifying power to DC before inverting the DC power to AC power such as 60 Hz. line voltage. This approach requires a converter that can sink current of opposite polarity to the output voltage. However, most converters cannot accommodate a non-unity power factor load. Additionally, for a given power level, in a single phase rectifier, the current pulses that comprise the ripple may be four to five times as large as the peak of the current waveform for an equivalent unity power factor load. Such current requires much larger conductors to minimize resistive losses. In polyphase rectifiers, the current peaks are not as large since smaller peaks occur more frequently, but the power quality is not as good because of large current transitions.
Both the cycloconverter and the devices for converting and inverting power use, transformer rectifier units that operate at line frequency chop power at low frequencies creating low frequency fundamental sinusoids thus requiring large and heavy transformers and large capacitors to store energy for smoothing peaks and filling valleys in the waveform. Introduction of such elements often adds reactive factors that affect the power factor. In such configurations the reactance causes the current to either xe2x80x9cleadxe2x80x9d the voltage or to xe2x80x9clagxe2x80x9d the voltage. Like the input solution, power conditioning is necessary for power-factor correction. Often this includes the use of synchronous motors spinning in xe2x80x9cno loadxe2x80x9d states. All of these solutions prove to be costly. The need for constant frequency power has justified these solutions.
There exists, then, an unmet need for a power unit for converting polyphase variable frequency AC power to constant frequency AC polyphase power while maintaining a unity power factor. Internal control of the magnitude of the voltage would further enhance the utility of such a power unit.
The inventive device exploits high frequency modulation of the signal to balance the power throughout the input voltage cycle. By modulating both the width and the amplitude of the power waveform, no power is lost and the power factor remains substantially the same as that of the load when the power factor is calculated at the input frequency.
The invention receives polyphase power of variable frequency and passes power of a set frequency. Relationships between phases are exploited according to known trigonometric identities. Each phase of the power is modulated distinctly until at the terminals of the transformer, where the output voltage represents the difference in potential pairs of terminals. To describe the invention here, the circuitry for handling a single phase of the power passed by the inventive device. For that purpose, an exemplary phase of the polyphase power supply is set forth here.
The present invention comprises a system and method for a direct conversion programmable power source controller. The invention controls three-phase power by accepting a reference signal; generating three modulating signals, each for one phase of the input power and having a frequency equal to the sum of the input frequency and the desired output frequency; accepting the three-phase electrical power from a wye connected power source; creating a high, chopping frequency for sampling; phase-angle-modulating the source signal at the chopping frequency according to each of the modulation signals. While in the high frequency state, the power passes easily through small two-winding transformers used either for isolation or for stepping up or stepping down the power. Once through the transformer, the invention then re-reverses the reversed power and chops a portion of each half-cycle in a manner that restores the pulse-width modulation of the power sinusoid by the modulating sinusoid. Reversing the polarity of each phase of electrical power at the phase-angle modulation frequency to reproduce the pulse-width modulated waveform at the output.
Filters on the output integrate the power waves before feeding the power to the load in delta. Between any two terminals of the delta connection, the output voltage presents as a well-formed sinusoid.
According to further aspects of the invention, the invention converts three-phase AC power to programmable frequency three-phase power without an intermediate DC stage and without large power storage devices.
The invention presents the source with the same power factor as the load.
According to the invention, conversion to high frequency reduces the size of necessary isolation transformers.
Also, inclusion of small high-frequency transformers allows correction for low-line conditions.
Further, the high-frequency conversion technique reduces the size of the input and output filters and allows those filters to produce near perfect sine waves.
Users can also optionally program output voltage and frequencies without component change by providing appropriate reference waveforms.
The invention also allows for bi-directional power flow thus to accommodate non-unity power factor (reactive), or braking or regenerative loads.
Wide separation of conversion and output frequencies eliminates phase errors due to input and output filters. The presence of a phase reference allows comparison with output waveforms for control loops. The control loops eliminate line disturbances such as phase-voltage imbalance and short-term spikes.
Further, the control process does not require digital signal processing. The inventive device may operate with a state machine with a look-up table for the desired waveforms, and a standard multiplying digital-to-analog converter.
The present invention provides means of converting three-phase power of variable frequency and amplitude to three-phase power of programmable and constant frequency and amplitude without an intermediate DC conversion.